This invention relates to high melting polyolefin copolymers suitable as thermoplastic elastomers and catalysts and methods for their synthesis. These olefin copolymers are characterized by low glass transition temperatures, melting points above about 90xc2x0 C., high molecular weights, and a narrow composition distribution between chains. The copolymers of the invention are novel reactor blends that can be sequentially frationated into fractions of differing crystallinities, said fractions nevertheless show compositions of comonomers which differ by less than 15% from the parent reactor blend. The invention also relates to a process for producing such copolymers by utilizing an unbridged metallocene catalyst that is capable of interconverting between states with different copolymerization characteristics.
Ethylene alpha-olefin copolymers are important commercial products. These copolymers find a particularly broad range of application as elastomers. There are generally three family of elastomers made from such copolymers. One class is typified by ethylene-propylene copolymers (EPR) which are saturated compounds, optimally of low crystallinity, requiring vulcanization with free-radical generators to achieve excellent elastic properties. Another type of elastomer is typified by ethylene-propylene terpolymers (EPDM), again optimally of low crystallinity, which contain a small amount of a non-conjugated diene such as ethylidene norbornene. The residual unsaturation provided by the diene termonomomer allows for vulcanization with sulfur, which then yields excellent elastomeric properties. Yet another class is typified by ethylene-alpha olefin copolymers of narrow composition distribution which possess excellent elastomeric properties even in the absence of vulcanization. For example U.S. Pat. No. 5,278,272, to Dow describes a class of substantially linear polyolefin copolymer elastomers with narrow composition distribution and excellent processing characteristics. (These latter class of elastomers are typified for example by the EXACTtm and ENGAGEtm brand products sold commercially by Exxon and Dow, respectively.) One of the limitations of the latter class of elastomers is the low melting temperature of these materials which limits their high temperature performance.
Hence it would be extremely advantageous to industry to produce copolymers of ethylene and alpha olefins which would show both elastomeric properties in the unvulcanized state and high melting points.
It is among the objects of this invention to provide methods of production of a class of novel polyolefin copolymers with a combination of interesting and useful physical characteristics, including a molecular weight distribution, Mw/Mn less than /=10, a narrow composition distribution,  less than /=15%, high melting point index, melting points greater than about 90xc2x0 C. and elastomeric properties. It is a further object of this invention to produce a novel family of crystallizable, high-melting polyolefin copolymers having a narrow composition distribution where the melting point of the polymer is greater than about 90xc2x0 C. It is a further object of this invention to produce a class of high-melting, multiblock, blend, and multiblockiblend polyolefin copolymer elastomers. These novel polymers are useful as elastomeric and/or thermoplastic materials as well as compatibilizers for other polyolefin blends.
We have unexpectedly found that it is possible to prepare high melting polyolefin elastomers of narrow composition distribution using novel unbridged metallocenes as olefin polymerization catalysts. For convenience, certain terms used throughout the specification are defined below (with xe2x80x9c less than /=xe2x80x9d or xe2x80x9c greater than /=xe2x80x9d meaning less than or equal to, or greater than or equal to):
a. xe2x80x9cMultiblockxe2x80x9d polymer or copolymer means a polymer comprised of multiple block sequences of monomer units where the structure or composition of a given sequence differs from that of its neighbor. Furthermore a multiblock copolymer as defined herein will contain a given sequence at least twice in every polymer chain.
b. The term xe2x80x9ccomposition distributionxe2x80x9d refers to the variation in comonomer composition between different polymer chains and can be described as a difference, in mole percent, of a given weight percent of a sample from the mean mole percent composition. The distribution need not be symmetrical around the mean; when expressed as a number, for example 15%, this shall mean the larger of the distributions from the mean.
c. As used herein, the term xe2x80x9celastomericxe2x80x9d refers to a material which tends to regain its shape upon extension, such as one which exhibits a positive power of recovery at 100, 200 and 300% elongation.
d. The term xe2x80x9cmelting point indexxe2x80x9d, also referred-to as MPI=Tm/Xc, means the ratio of the melting point of the copolymer, Tm, to the mole fraction of the crystallizable component, Xc. By crystallizable component, we mean a monomer component whose homopolymer is a crystalline polymer. The melting point is taken as a maximum in a melting endotherm, as determined by differential scanning calorimetry.
The copolymers of the present invention have the following characteristics:
(a) a mole fraction of crystallizable component Xc from 30-99%;
(b) a molecular weight distribution Mw/Mn less than /=10; and
(c) melting points above about 90xc2x0 C.;
which copolymers comprise from 0-70% by weight of an ether soluble fraction, and from 0-95% of a hexanes soluble fraction which can exhibit a melting range up to about 125xc2x0 C., and from 0-95% of a hexanes insoluble fraction which can exhibit a melting range up to about 142xc2x0 C.
The copolymers of the present invention in one embodiment can be characterized as reactor blends in that they can be fractionated into fractions of differing degrees of crystallinity and differing melting points. Nevertheless, the comonomer composition of the various fractions of the copolymers are within 15% of the composition of the resultant polymer product produced in the reactor.
The melting points of the copolymers of the present invention are high, typically above 90xc2x0 C. and the melting point indices, Tm/Xc are also high, typically above 80 xc2x0 C. and preferably above 115xc2x0 C. The fractions can also exhibit high melting point indices. For example, it is possible to isolate a hexanes soluble fraction from the copolymers of the present invention that exhibits a melting point as high as 115xc2x0 C. and a melting point index as high as 160xc2x0 C. The glass transition temperatures of the copolymers are low, typically less than xe2x88x9220xc2x0 C. and preferably below xe2x88x9250xc2x0 C.
The molecular weights of the polymers of the present invention can be quite high, with weight average molecular weights in excess of Mw=1,000,000 readily obtained and molecular weights as high as 2,000,000 observed. The molecular weight distributions of the copolymers are typically Mw/Mn less than /=10, preferably Mw/Mn less than /=8 and most preferably  less than /=6.
In one embodiment, the copolymers of the present invention exhibit useful elastomeric properties. They can be used in a variety of applications typical of amorphous or partially crystalline elastomers and as compatibilizers for copolymer blends.
While not wishing to be bound by theory, it is believed that in the process of the invention, different active species of the catalyst are in equilibrium during the construction of the copolymer chains. This is provided for in the present invention by a class of unbridged metallocenes that are capable of isomerizing between states that have different copolymerization characteristics during the polymerization process. This process can thus lead to multiblock copolymers or copolymer blends where the blocks or components of the blends have different compositions of comonomers.
One embodiment of the invention includes metallocene catalysts which are able to interconvert between states whose coordination geometries are different. Thus, the invention includes selecting the substituents of the metallocene cyclopentadienyl ligands so that the rate of interconversion of the two states is within several orders of magnitude of the rate of formation of a single polymer chain. That is, if the rate of interconversion between states of the catalyst, ri, is greater than the rate of formation of an individual polymer chain, rf, on average, the polymer resulting from use of the inventive process and catalysts can be characterized as multiblock (as defined above). If the rate of interconversion is less than the rate of formation, the result is a polymer blend. Where the rates are substantially balanced, the polymer can be characterized as a mixture of blend and multiblock. There may be a wide range of variations and intermediate cases amongst these three exemplars.
The nature of the substituents on the cyclopentadienyl ligands is critical; the substitution pattern of the cyclopentadienyl ligands should be such that the coordination geometries are different in order to provide for different reactivities toward ethylene and other alpha olefins while in the two states (see FIG. 1) and that the rate of interconversion of the states of the catalyst are within several orders of magnitude of the rate of formation of a single chain.
A further embodiment includes metallocene catalysts which are able to interconvert between more than two states whose coordination geometries are different. This is provided for by metallocenes with cyclopentadienyl-type ligands substituted in such a way that more than two stable states of the catalyst have coordination geometries that are different, for example, a catalyst with four geometries is illustrated in one embodiment in FIG. 2.
According to the process of this invention, the properties of the copolymers can be controlled by changing the nature of the cyclopentadienyl units on the catalysts, by changing the nature of the metal atom in the catalyst, by changing the nature of the comonomers and the comonomer feed ratio, and by changing the temperature.
The molecular weights of the polymers produced with the catalysts of the invention are very high. The molecular weight of the polymer product can be controlled, optionally, by controlling the temperature or by adding any number of chain transfer agents such as hydrogen or metal alkyls, as is well known in the art.
The catalyst system of the present invention consists of the transition metal component metallocene in the presence of an appropriate cocatalyst. In broad aspect, the transition metal compounds have the formula: 
in which M is a Group 3, 4 or 5 Transition metal, a Lanthanide or an Actinide, X and Xxe2x80x2 are the same or different uninegative ligands, such as but not limited to hydride, halogen, hydrocarbyl, halohydrocarbyl, amine, amide, or borohydride substituents (preferably halogen, alkoxide, or C1 to C7 hydrocarbyl), and L and Lxe2x80x2 are the same or different polynuclear hydrocarbyl, silahydrocarbyl, phosphahydrocarbyl, azahydrocarbyl, arseni-hydrocarbyl or borahydrocarbyl rings, typically a substituted cyclo-pentadienyl ring or heterocyclopentadienyl ring, in combination with an appropriate cocatalyst. Exemplary preferred Transition Metals include Titanium, Haffiium, Vanadium, and, most preferably, Zirconium. An exemplary Group 3 metal is Yttrium, a Lanthanide is Samarium, and an Actinide is Thorium.
Preferably L and Lxe2x80x2 have the formula: 
where R1, R2, R3, R9, and R10 may be the same or different hydrogen, alkyl, alkylsilyl, aryl, benzyl, alkoxyalkyl, alkoxyaryl, alkoxysilyl, aminoalkyl, aminoaryl, branched alkyl, or substituted alkyl, alkylsilyl or aryl substituents of 1 to about 30 carbon atoms.
Ligands of this general structure include substituted cyclopentadienes. Other ligands L and Lxe2x80x2 of Formula 2 for the production of propylene-ethylene copolymers include substituted cyclopentadienes of the general formula: 
where R2-R10 have the same definition as R1, R2, R3, R9, and R10 above. Preferred cyclopentadienes of Formula 3 include 3,4-dimethyl-1-phenyl-1,3-cyclopentadiene (R2xe2x95x90R3xe2x95x90CH3, R6xe2x95x90H); 3,4-dimethyl-1-p-tolyl-1,3-cyclopentadiene (R2xe2x95x90R3xe2x95x90CH3, R6xe2x95x90CH3); 3,4,-dimethyl-1-(3,5-bis(trifluoromethyl)phenyl)-1,3-cyclopentadiene (R2xe2x95x90R3xe2x95x90CH3, R6xe2x95x90CF3); and 3,4-dimethyl-1-(4-tert-butylphenyl)-1,3-cyclo-pentadiene (R2xe2x95x90R3xe2x95x90CH3, R6xe2x95x90tBu).
Alternately preferred L and Lxe2x80x2 of Formula 1 include ligands of Formula 2 wherein R1 is an aryl group, such as a substituted phenyl, biphenyl, or naphthyl group, and R2 and R3 are connected as part of a ring of three or more carbon atoms. Especially preferred for L or Lxe2x80x2 of Formulas 1-3 for producing the copolymers of this invention are substituted indenyl ligands, more particularly 2-arylindene of formula: 
where R4-R14 may be the same or different hydrogen, halogen, aryl, benzyl, hydrocarbyl, silahydrocarbyl, or halohydrocarbyl substituents. That is, R1 of Formula 2 is R4-R8-substituted benzene, and R2, R3 are cyclized in a 6-carbon ring to form the indene moiety.
Particularly preferred 2-aryl indenes include at present as preferred best mode compounds: 2-phenylindene (Ligand C); 1-methyl-2-phenyl indene (Ligand D); 2-(3,5-dimethylphenyl)indene; 2-(3,5-bis-trifluoromethylphenyl)indene (Ligand A); 1-methyl-2-(3,5-bis-trifluoromethylphenyl)indene (Ligand B); 2-(3,5-bis-tertbutylphenyl)indene; 1-methyl-2-(3,5-bis-tertbutylphenyl)indene; 2-(3,5-bis-trimethyl- silylphenyl)indene; 1-methyl-2-(3,5-bis-trimethylsilylphenyl)indene; 2-(4-fluorophenyl)indene; 2-(2,3,4,5-tetrafluorophenyl)indene; 2-(2,3,4,5,6-pentafluorophenyl)indene; 2-(1-naphthyl)indene; 2-(2-naphthyl)indene; 2-[(4-phenyl)phenyl]indene; 2-[(3-phenyl)phenyl]indene; 1-benzyl-2-phenylindene; and 1-benzyl-2(3,5-bis-tertbutylphenyl)indene (Ligand E).
Preferred metallocenes according to the present invention include both bis ligand (two identical ligands) and mixed ligands (the ligands are not identical) metallocenes having any combination of the above aryl indenes (and their sterically functionally hindering equivalents), such as but not limited to:
bis(2-phenylindenyl)zirconium dichloride;
bis(2-phenylindenyl)zirconium dimethyl;
bis(1-methyl-2-phenylindenyl)zirconium dichloride;
bis(1-methyl-2-phenylindenyl)zirconium dimethyl;
bis[2-(3,5-dimethylphenyl)indenyl]zirconium dichloride;
bis[2-(3,5-bis-trifluoromethylphenyl)indenyl]zirconium dichloride;
bis[2-(3,5-bis-tertbutylphenyl)indenyl]zirconium dichloride;
bis[2-(3,5-bis-trimethylsilylphenyl)indenyl]zirconium dichloride;
bis[2-(4,-fluorophenyl)indenyl]zirconium dichloride;
bis[2-(2,3,4,5,-tetraflorophenyl)indenyl]zirconium dichloride;
bis(2-(2,3,4,5,6-pentafluorophenyl)indenyl])zirconium dichloride;
bis[2-(1-naphthyl)indenyl]zirconium dichloride;
bis(2-(2-naphthyl)indenyl])zirconium dichloride;
bis(2-[(4-phenyl)phenyl]indenyl])zirconium dichloride;
bis[2-[(3-phenyl)phenyl]indenyl]zirconium dichloride;
(pentamethylcyclopentadienyl)(1-methyl-2-phenylindenyl)zirconium dichloride;
(pentamethylcyclopentadienyl)(2-phenylindenyl)zirconium dichloride;
(pentamethylcyclopentadienyl)(1-methyl-2-phenylindenyl)zirconium dimethyl;
(pentamethylcyclopentadienyl)(2-phenylindenyl)zirconium dimethyl;
(cyclopentadienyl)(1-methyl-2-phenylindenyl)zirconium dichloride;
(cyclopentadienyl)(2-phenylindenyl)zirconium dichloride;
(cyclopentadienyl)(1-methyl-2-phenylindenyl)zirconium dimethyl;
(cyclopentadienyl)(2-phenylindenyl)zirconium dimethyl;
(1-methyl-2-phenylindenyl)(2-phenylindenyl)zirconium dichloride (i.e., Ligands D and C, Metallocene 6, Table 3);
(1-methyl-2-phenylindenyl)[2-(3,5-bis-trifluoromethylphenyl)indenyl]zirconium dichloride (i.e., Ligands D and A, Metallocene 8, Table 3);
[1-methyl-2-(3,5-bis-trifluoromethylphenyl)indenyl](2-phenylindenyl)zirconium dichloride (i.e., Ligands B and C, Metallocene 7, Table 3);
[1-methyl-2-(3,5-bis-trifluoromethylphenyl)indenyl][2-(3,5-bis-trifluoromethyphenyl)indenyl]zirconium dichloride (i.e., Ligands B and A, Metallocene 9, Table 3); (1-methyl-2-phenylindenyl)[2-(3,5-bis-tertbutylphenyl)indenyl]zirconium dichloride;
(2-phenylindenyl)(1-benzyl-2-phenylindenyl)zirconium dichloride;
(2-phenylindenyl)[2-(3,5-bis-trifluoromethylphenyl)indenyl]zirconium dichloride (i.e., Ligands C and A);
[2-(3,5-bis-trifluoromethylphenyl)indenyl][1-benzyl-2-(3,5-bis-tertbutylphenyl)indenyl]zirconium dichloride (i.e., Ligands A and E);
(2-phenylindenyl)][1-benzyl-2-(3,5-bis-tertbutylphenyl)indenyl]zirconium dichloride (i.e., Ligands C and E, Metallocene 11);
and the corresponding hafnium compounds such as:
bis(2-phenylindenyl)hafnium dichloride;
bis(2-phenylindenyl)hafnium dimethyl;
bis(1-methyl-2-phenylindenyl)hafnium dichloride;
bis(1-methyl-2-phenylindenyl)hafnium dimethyl;
bis[2-(3,5-dimethylphenyl)indenyl]hafnium dichloride;
bis[2-(3,5-bis-trifluoromethyphenyl)indenyl]hafnium dichloride;
bis[2-(3,5-bis-tertbutylphenyl)indenyl]hafnium dichloride;
bis[2-(3,5-bis-trimethylsilylphenyl)indenyl]hafnium dichloride;
bis[2,(4-fluorophenyl)indenyl]hafnium dichloride;
bis[2-(2,3,4,5-tetrafluorophenyl)indenyl]hafnium dichloride;
bis[2-(2,3,4,5,6-pentafluorophenyl)indenyl]hafnium dichloride;
bis[2-(1-naphthyl)indenyl]hafnium dichloride;
bis[2-(2-naphthyl)indenyl]hafnium dichloride;
bis(2-((4-phenyl)phenyl)indenyl])hafnium dichloride;
bis[2-[(3-phenyl)phenyl]indenyl]hafnium dichIoride;
(pentamethylcyclopentadienyl)(1-methyl-2-phenylindenyl)hafnium dichloride;
(pentamethylcyclopentadienyl)(2-phenylindenyl)hafnium dichloride;
(pentamethylcyclopentadienyl)(1-methyl-2-phenylindenyl)hafnium dimethyl;
(pentamethylcyclopentadienyl)(2-phenylindenyl)hafnium dimethyl;
(cyclopentadienyl)(1-methyl-2-phenylindenyl)hafnium dichloride;
(cyclopentadienyl)(2-phenylindenyl)hafnium dichloride;
(cyclopentadienyl)(1-methyl-2-phenylindenyl)hafnium dimethyl;
(cyclopentadienyl)(2-phenylindenyl)hafnium dimethyl;
(1-methyl-2-phenylindenyl)(2-phenylindenyl)hafnium dichloride (Ligands D and C);
(1-methyl-2-phenylindenyl)[2-(3,5-bis-trifluoromethylphenyl)indenyl]hafnium dichloride (Ligands D and A);
[1-methyl-2-(3,5-bis-trifluoromethylphenyl)indenyl](2-phenylindenyl)hafnium dichloride (Ligands B and C);
[1-methyl-2-(3,5-bis-trifluoromethylphenyl)indenyl][2-(3,5-bis-trifluoromethylphenylindenyl]hafnium dichloride (Ligands B and A);
(1-methyl-2-phenylindenyl)[2-(3,5-bis-tertbutylphenyl)indenyl]hafnium dichloride;
(2-phenylindenyl)(1-benzyl-2-phenylindenyl)hafnium dichIoride;
(2-phenylindenyl)[2-(3,5-bis-trifluoromethylphenyl)indenyl]hafnium dichloride (i.e., Ligands C and A);
[2-(3,5-bis-trifluoromethylphenyl)indenyl][1-benzyl-2-(3,5-bis-tertbutylphenyl)indenyl]hafnium dichloride (i.e., Ligands A and E);
(2-phenylindenyl)][1-benzyl-2-(3,5-bis-tertbutylphenyl)indenyl]hafnium dichloride (i.e., Ligands C and E); and the like.
Other metallocene catalyst components of the catalyst system according to the present invention include:
bis(3,4-dimethyl-1-phenylcyclopentadienyl)zirconium dichloride;
bis(3,4-dimethyl-1-p-tolylcyclopentadienyl)zirconium dichloride;
bis(3,4-dimethyl-1-(3,5 bis(trifluoromethyl)phenyl)cyclopentadienyl)zirconium dichloride;
bis(3,4-dimethyl-1-(4-tert-butylphenyl)cyclopentadienyl)zirconium dichloride;
(3,4-dimethyl-1-phenyl-1,3-cyclopentadiene)(3,4-dimethyl-1-p-tolylcyclopentadienyl)zirconium dichloride;
(3,4-dimethyl-1-phenylcyclopentadienyl)(3,4-dimethyl-1-(3,5 bis(trifluoromethyl)phenyl)cyclopentadienyl)zirconium dichloride;
(3,4-dimethyl-1-phenylcyclopentadienyl)(3,4-dimethyl-1-(4-tert-butylphenyl)cyclopentadienyl)zirconium dichloride;
and the corresponding hafnium compounds, such as:
bis(3,4-dimethyl-1-phenylcyclopentadienyl)hafnium dichloride;
bis(3,4-dimethyl-1-p-tolylcyclopentadienyl)hafnium dichloride;
bis(3,4-dimethyl-1-(3,5 bis(trifluoromethyl)phenyl)cyclopentadienyl)haffium dichloride;
bis(3,4-dimethyl-1-(4-tert-butylphenyl)cyclopentadienyl)hafnium dichloride;
(3,4-dimethyl-1-phenylcyclopentadienyl)(3,4-dimethyl-1-p-tolylcyclopentadienyl)hafnium dichloride;
(3,4-dimethyl-1-phenylcyclopentadienyl)(3,4-dimethyl-1-(3,5 bis(trifluoromethyl)phenyl)cyclopentadienyl)hafnium dichloride;
(3,4-dimethyl-1-phenylcyclopentadienyl)(3,4-dimethyl-1-(4-tert-butylphenyl)cyclopentadienyl)hafnium dichloride; and the like.
It should be understood that other unbridged, rotating, non-rigid, fluxional metallocenes may be employed in the methods of this invention, including those disclosed in our above-identified Provisional application, which is hereby incorporated by reference to extent needed for support.
The Examples disclose a method for preparing the metallocenes in high yield. Generally, the preparation of the metallocenes consists of forming the indenyl ligand followed by metallation with the metal tetrahalide to form the complex.
Appropriate cocatalysts include alkylaluminum compounds, methylaluminoxane, or modified methylaluminoxanes of the type described in the following references: U.S. Pat. No. 4,542,199 to Kaminsky, et al,; Ewen, J. Am. Chem. Soc., 106 (1984), p. 6355; Ewen, et al., J. Am. Chem. Soc. 109 (1987) p. 6544; Ewen, et al., J. Am. Chem. Soc. 110 (1988), p. 6255; Kaminsky, et al, Angew. Chem., Int. Ed. Eng. 24 (1985), p. 507. Other cocatalysts which may be used include Lewis or protic acids, such as B(C6F5)3 or (PhNMe2H)+B(C6F5)4xe2x88x92, which generate cationic metallocenes with compatible non-coordinating anions in the presence or absence of alkyl-aluminum compounds. Catalyst systems employing a cationic Group 4 metallocene and compatible non-coordinating anions are described in European Patent Applications 277,003 and 277,004 filed on 27.01.88 by Turner, et al.; European Patent Application 427,697-A2 filed on Sep. 10, 1990 by Ewen, et al.; Marks, et al., J. Am. Chem. Soc., 113 (1991), p. 3623; Chien, et al., J. Am. Chem. Soc., 113 (1991), p. 8570; Bochmann et al., Angew. Chem. Intl., Ed. Engl. 7 (1990), p. 780; and Teuben et al., Organometallics, 11 (1992), p. 362, and references therein.
The catalysts of the present invention comprise un-bridged, non-rigid, fluxional metallocenes which can change their geometry with a rate that is within several orders of magnitude of the rate of formation of a single polymer chain, on average. In accordance with the invention, the relative rates of interconversion and of formation can be controlled by selecting the substituents (or absence thereof) of the cyclopentadienyl ligands so that they can alternate in structure between states of different coordination geometries which have different selectivity toward a particular comonomer.
In one of many embodiments, these catalyst systems can be placed on a suitable support such as silica, alumina, or other metal oxides, MgCl2 or other supports. These catalysts can be used in the solution phase, in slurry phase, in the gas phase, or in bulk monomer. Both batch and continuous polymerizations can be carried out. Appropriate solvents for solution polymerization include liquified monomer, and aliphatic or aromatic solvents such as toluene, benzene, hexane, heptane, diethyl ether, as well as halogenated aliphatic or aromatic solvents such as CH2Cl2, chlorobenzene, fluoro benzene, hexaflourobenzene or other suitable solvents. Various agents can be added to control the molecular weight, including hydrogen, silanes and metal alkyls such as diethylzinc.
The metallocenes of the present invention, in the presence of appropriate cocatalysts, are useful for the homo-polymerization and co-polymerization of alpha-olefins, such as propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, and combinations thereof, and of copolymerization with ethylene. The polymerization of olefins is carried out by contacting the olefin(s) with the catalyst systems comprising the transition metal component and in the presence of an appropriate cocatalyst, such as an aluminoxane, a Lewis acid such as B(C6F5)3, or a protic acid in the presence of a non-coordinating counterion such as B(C6F5)4xe2x88x92.
The metallocene catalyst systems of the present invention are particularly useful for the polyrnerization of ethylene and alpha-olefin comonomers as well as alpha-olefin monomer mixtures to produce co-polymers with novel elastomeric properties. The properties of elastomers are characterized by several variables. The tensile set (TS) is the elongation remaining in a polymer sample after it is stretched to an arbitary elongation (e.g. 100% or 300%) and allowed to recover. Lower set indicates higher elongational recovery. Stress relaxation is measured as the decrease in stress (or force) during a time period (e.g. 30 sec. or 5 min.) that the specimen is held at extension. There are various methods for reporting hysteresis during repeated extensions. In the present application, retained force is measured as the ratio of stress at 50% elongation during the second cycle recovery to the initial stress at 100% elongation during the same cycle. Higher values of retained force and lower values of stress relaxation indicate stronger recovery force. Better general elastomeric recovery properties are indicated by low set, high retained force and low stress relaxation.